Realtime thread priorities

David Xu davidxu at
Wed Dec 15 01:40:12 UTC 2010

John Baldwin wrote:
> On Monday, December 13, 2010 8:30:24 pm David Xu wrote:
>> John Baldwin wrote:
>>> On Sunday, December 12, 2010 3:06:20 pm Sergey Babkin wrote:
>>>> John Baldwin wrote:
>>>>> The current layout breaks up the global thread priority space (0 - 255) 
>>> into a
>>>>> couple of bands:
>>>>>   0 -  63 : interrupt threads
>>>>>  64 - 127 : kernel sleep priorities (PSOCK, etc.)
>>>>> 128 - 159 : real-time user threads (rtprio)
>>>>> 160 - 223 : time-sharing user threads
>>>>> 224 - 255 : idle threads (idprio and kernel idle procs)
>>>>> If we decide to change the behavior I see two possible fixes:
>>>>> 1) (easy) just move the real-time priority range above the kernel sleep
>>>>> priority range
>>>> Would not this cause a priority inversion when an RT process
>>>> enters the kernel mode?
>>> How so?  Note that timesharing threads are not "bumped" to a kernel sleep 
>>> priority when they enter the kernel either.  The kernel sleep priorities are 
>>> purely a way for certain sleep channels to cause a thread to be treated as 
>>> interactive and give it a priority boost to favor interactive threads.  
>>> Threads in the kernel do not automatically have higher priority than threads 
>>> not in the kernel.  Keep in mind that all stopped threads (threads not 
>>> executing) are always in the kernel when they stop.
>> I have requirement to make a thread running in kernel has more higher
>> priority over a thread running userland code, because our kernel
>> mutex is not sleepable which does not like Solaris did, I have to use
>> semaphore like code in kern_umtx.c to lock a chain, which allows me
>> to read and write user address space, this is how umtxq_busy() did,
>> but it does not prevent a userland thread from preempting a thread
>> which locked the chain, if a realtime thread preempts a thread
>> locked the chain, it may lock up whole processes using pthread.
>> I think our realtime scheduling is not very useful, it is too easy
>> to lock up system.
> Users are not forced to use rtprio.  They choose to do so, and they have to
> be root to enable it (either directly or by extending root privileges via
> sudo or some such).  Just because you don't have a use case for it doesn't
> mean that other people do not.  Right now there is no way possible to say
> that a given userland process is more important than 'sshd' (or any other
> daemon) blocked in poll/select/kevent waiting for a packet.  However, there
> are use cases where other long-running userland processes are in fact far
> more important than sshd (or similar processes such as getty, etc.).
You still don't answer me about how to avoid a time-sharing thread
holding a critical kernel resource which preempted by a user RT thread,
and later the RT thread requires the resource, but the time-sharing
thread has no chance to run because another RT thread is dominating
the CPU because it is doing CPU bound work, result is deadlock, even if
you know you trust your RT process, there are many code which were
written by you, i.e the libc and any other libraries using threading
are completely not ready for RT use.
How ever let a thread in kernel have higher priority over a thread
running userland code will fix such a deadlock in kernel.

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